Automatic exposure control for a slow scan vidicon

ABSTRACT

The present invention provides an automatic exposure control circuit for a vidicon camera tube wherein the target element of the vidicon is utilized as the &#39;&#39;&#39;&#39;heart&#39;&#39;&#39;&#39; of its own exposure control. More specifically, the target element output signal is integrated to produce a voltage whose magnitude is proportional to average image intensity; which proportionate or analog voltage is then compared to a reference voltage preselected in accordance with desired video output level. The comparator output is then used as instantaneous feedback to operate the vidicon shutter when the proportionate or analog voltage signal comes into agreement with the reference voltage.

United States Patent Inventor Thomas Thompson Silver Spring, Md. Appl.No. 690,808 Filed Dec. 15, 1967 Patented Apr. 27, 1971 Assignee e Un tedSta e of America asremsen by the Secretary of the Navy AUTOMATICEXPOSURE CONTROL FOR A SLOW SCAN VIDICON Primary Examiner-Richard MurrayAssistant Examiner-Richard P. Lange Attorneys-R. S. Sciascia and J. A.Cooke ABSTRACT: Thepresent invention provides an automatic exposurecontrol circuit for a vidicon camera tube wherein the target element ofthe vidicon is utilized as the heart" of its schlms l own exposurecontrol. More specifically, the target element US. Cl 178/7.2 outputsignal is integrated to produce a voltage whose magllltr Cl v H nitudeis proportional to average image intensity; which pro- Field of Searchl78/7.2, 7.2 portionate or analog voltage is then compared to areference voltage preselected in accordance with desired video outputlevel. The comparator output is then used as instantaneous Refemnm cuedfeedback to operate the vidicon shutter when the propor- UNITE STA S ENS tionate or analog voltage signal comes into agreement with the2,930,929 3/1960 Sheldon l78/7.2(E) reference voltage.

INPUT SHUTTER EITHER MECHANICAL UGHT OR ELECTRONIC) 1 I I8 I VIDICON I0I I l REFERENCE l7 VOLTAGE b COMPARATOR e INTEGRATOR TARGET VOLTAGEPATENTEU mm :sn

INVENTOR. Thomas Thompson 0N mw4PJO wozmmwmwm 2 v v AIIIIII. SEQESd mo43.5182 5:5: mmPFDIm k III'III AUTOMATIC EXPOSURE CONTROL FOR A SLOWSCAN VIDICON BACKGROUND OF THE INVENTION The present invention generallyrelates to automatic exposure control circuitry for television cameratubes and more particularly, it relates to automatic exposure controlcircuitry for any slow scan vidicon camera tube, including the so-calledSEC vidicon.

Much research effort is currently being expended in the area oftelevision satellite work; i.e., the mounting of a television system ona near earth satellite for the purpose of obtaining television picturesof the earth from the orbiting satellite. Vidicon camera tubes areparticularly attractive for this television satellite application andthe need thus exists for circuitry a capable of insuring that the videooutput level from the camera tube aboard the satellite is maintained atan acceptable level. This is particularly so in view of the fact thatthe input light intensity received at the satellite may be subject towide range variations. Without an automatic exposure control for thetelevision camera tube in the satellite, unacceptable video informationis often obtained.

Although many exposure control methods and associated apparatus havebeen proposed heretofore, they are unsuitable or unadapted for exposurecontrol of a slow scan vidicon. In other words, the prior art isrestricted in one sense to providing exposure control for so-calledreturned beam-type camera tubes, such as the image orthicon. On theother hand, certain of the prior art control systems are peakexposure-type systems whereinthe peak image intensity is sensed andsubsequently utilized for controlling the exposure of the camera I tube.This latter type of system is unadaptable to providing average imageintensity control by adjustment of the video output level in that thepeak-type system operates on a frameto-frame basis and implies that theinput light intensity information content in the scene-being viewed willremain relatively constant from frame to frame.

It is proposed in accordance with the present invention to provideautomatic exposure control circuitry, particularly adapted for use withslow scan vidicon camera tubes wherein the instantaneous image intensityat the vidicon target elements is sensed and then averaged to produce asignal proportionate to the average image intensity seen by the cameratube. The analog of the average image intensity is then subsequentlyutilized to control the camera tube shutter, which may be eithermechanical or electronic, in such a manner that the shutter is closedwhen the video output on the camera tube reaches a proper and acceptablelevel. In this manner, the video output from the camera tube ismaintained relatively constant and therefore suitable for use in atelevision satellite system wherein, as previously mentioned, the systemis particularly susceptible to variations in the intensity of the inputimage viewed by the satellite-home camera tube.

One object of the present invention is therefore to provide an improvedautomatic exposure control system for slow scan vidicon camera tubes.

A further object of the present invention is to provide a control systemcapable of controlling the amount of exposure to which a slow scanvidicon camera tube is subjected in response to the average video outputlevel of the camera tube.

A still further object of the present invention is to provide anautomatic exposure control system for a slow scan vidicon camera tubewherein the target element of the camera tube forms an essential part ofthe camera tubes exposure control system, thus obviating the need foradditional equipment such as well-known beam splitters, etc.

Other objects, purposes and characteristic features of the presentinvention will in part be pointed out as the description of theinvention progresses and in part be obvious from the accompanyingdrawing which illustrates one embodiment of the proposed automaticexposure control circuit of the present invention, as adapted to controlthe exposure of a vidicon camera tube.

Referring now to the drawing, the automatic exposure control circuitry,proposed in accordance with the present invention is particularlyadapted for controlling theexposure of a vidicon camera tube,represented at 10. As is well known to those skilled in the art, thetypical slow scan vidicon camera tube includes a target element 11 whichcomprises a semiconductor member 12 mounted on a conductive plate member13. This target element 11 is directly exposed to the input light imageused by the vidicon camera tube 10. In the so-called SEC vidicon, theinput light image is first converted to an electronic pattern at aphotocathode disposed in front of the target element and the target isthen exposed to the electronic image. In the SEC vidicon, the conductiveplate 13 is a thin layer of aluminum.

As shown in the drawing, the conductive plate 13 of the target element11 is connected, through a suitable resistor 14 to a source of so-calledtarget voltage 15. During operation, electrons leak off thesemiconductor material 12 through the resistor 14 from regions wherelight (in the normal slow scan vidicon) or where the electronic image(in the SEC vidicon) strikes the target element. Therefore, the numberof leakage electrons; i.e., leakage current, which flow through theresistor 14 is proportional to the instantaneous light intensity orelectron image intensity to which the target element 11 is sub jected.This in turn is proportionate to the input light intensity viewed by thevidicon camera tube 10.

It is proposed in accordance with the present invention to integrate theleakage current flowing through the target resistor 14 and therebyobtain a measure of the average light intensity being viewed by thevidicon camera tube 10. To accomplish this, a suitable integratorcircuit 16 is adapted to be connected across the target resistor 14, bythe switch 17, to integrate or average the target leakage current fromthe vidicon 10. The switch 17 is controlled in turn by the operation ofthe camera tube shutter 18, which can be either mechanical orelectronic, in such a manner that the switch 17 is closed, to completethe connection to the integrator 16, when the camera tube shutter 18 isin its open position. As is well known to those skilled in the art,electronic shuttering of the vidicon camera tube 10 can be accomplishedby removing the acceleration high voltage from the camera tube 10. Theelectronic or mechanical shuttering action provided by shutter 18 isdiagrammatically represented in the drawing by the dashed control line18a extending between the shutter 18 and the INPUT LIGHT.

The integrator circuit 16 produces an output voltage signal representingthe average or integrated value of the leakage current; i.e., videooutput level, from the vidicon 10. This integrator output voltage signalis applied to a suitable comparator circuit 19, along with a preselectedreference voltage signal preset in accordance with the desired videooutput level from the vidicon 10. The comparator 19 operates in aconventional manner, to produce an output signal when the integratoroutput voltage signal becomes equal to the preset reference voltagesignal applied at input 20. More specifically, the comparator output isapplied to the shutter mechanism 18 and is effective to operate theshutter to its closed position when the video output level from thevidicon reaches its desired value, as preset by the reference voltage20.

From the foregoing description, it will thus be seen that the proposedautomatic exposure control circuitry of the present invention providesan effective means for accurately conteachings Therefore, it should beunderstood at this time, that within the scope of the appended claims,the invention may be practiced otherwise than as specifically described.

I claim:

1. In combination:

a vidicon camera tube having a target element and being equipped with ashutter having open and closed positions, said target element beingexposed to an image when said shutter is in its open position,

circuit means operably connected to said target element and effectiveduring a frame interval when said target element is exposed to an imageto produce an analog output signal proportionate to the average imageintensity at said target element, and

control means responsive to said analog output signal for closing saidshutter during the same frame interval to terminate exposure of saidtarget element to said image when said output signal exceeds apredetermined value.

2. The combination specified in claim 1, wherein said control meansincludes:

a reference signal source,

comparator means operably connected to compare the output signal fromsaid circuit means with said reference signal, and I means operablyconnected to said comparator means for closing said shutter when saidanalog output signal from said circuit means exceeds said referencesignal.

3. The combination specified in claim 2, wherein said reference signalis preset in accordance with the desired video output level for saidvidicon camera tube.

4. The combination specified in claim 1, wherein said circuit meansincludes:

a resistor connected in circuit series with said vidicon target elementand across which is developed a voltage signal proportionate to theinstantaneous image intensity at said target element when said shutteris open, and

integrator means having its input connected across said resistor forintegrating said resistor voltage signal.

5. The combination specified in claim 4, further including a switchmeans responsive to the condition of said shutter and adapted to closethe connection of said integrator input across said resistor when saidshutter is open.

1. In combination: a vidicon camera tube having a target element andbeing equipped with a shutter having open and closed positions, saidtarget element being exposed to an image when said shutter is in itsopen position, circuit means operably connected to said target elementand effective during a frame interval when said target element isexposed to an image to produce an analog output signal proportionate tothe average image intensity at said target element, and control meansresponsive to said analog output signal for closing said shutter duringthe same frame interval to terminate exposure of said target element tosaid image when said output signal exceeds a predetermined value.
 2. Thecombination specified in claim 1, wherein said control means includes: areference signal source, comparator means operably connected to comparethe output signal from said circuit means with said reference signal,and means operably connected to said comparator means for closing saidshutter when said analog output signal from said circuit means exceedssaid reference signal.
 3. The combination specified in claim 2, whereinsaid reference signal is preset in accordance with the desired videooutput level for said vidicon camera tube.
 4. The combination specifiedin claim 1, wherein said circuit means includes: a resistor connected incircuit series with said vidicon target element and across which isdeveloped a voltage signal proportionate to the instantaneous imageintensity at said target element when said shutter is open, andintegrator means having its input connected across said resistor forintegrating said resistor voltage signal.
 5. The combination specifiedin claim 4, further including a switch means responsive to the conditionof said shutter and adapted to close the connection of said integratorinput across said resistor when said shutter is open.